Change ambiguous list inline action "view" to "show".

Inline action `view` is very ambiguous, becuase we have route `show`, `ShowMapper`, `configureShowFields`. It's more clear if we have inline action called `show`.

Before:

        $listMapper
            ->add('_action', 'actions', array(
                'actions' => array(
                    'view' => array(),
                )
            ))
        ;

After:

        $listMapper
            ->add('_action', 'actions', array(
                'actions' => array(
                    'show' => array(),
                )
            ))
        ;

This change is backward compatible (old `CRUD/list__action_view.html.twig ` template is not deleted).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1619
| License       | MIT
| Doc PR        | -